Rollout is a form of gear ratio that goes a step further and takes tire diameter into account. It's basically defined as how far the car will roll with one revolution of the motor. So a car with a 1.6" rollout will go 1.6" for every revolution of the motor. As foam tires wear they get smaller and you have to go to larger pinions to keep the same rollout. I don't recall the formula off the top of my head but i'm sure someone else will chime in with it. I have rollout charts that I made in excel with the spur gear size as a constant, the pinion size on one axis, and the tire size on the other. This way you can know at a glance what rollout you are running, how much you have to change with different tires, etc. When I get home I will post my charts here.

-James